Abstract : The following live, attenuated dengue virus vaccines strains were tested for their respective immunogonicity in rhesus monkeys: dengue-1 (TP79-56, code TSI-GSD216 and 45AZ-5, code No. TSI-GSD214; dengue 3 (CH53489, clone 24/28); and dengue-4 (341750 Carib). It was found that 417 MID50 (mosquito infectious dose 50) and 116 MID50 were required for seroconversion of 50% of the animals with the 45AZ5 and the TP79-56 vaccine, respectively. Upon challenge with the homologous wild type virus, dengue-1 immune animals showed a typical secondary antibody response. A dose of 2.9 MID50 (dengue-3 vaccine) and of 0.158 MID50 (parent dengue-3 virus) seroconverted 50% of the animals. Experimental dat showed that the DEN-4 vaccine was highly attenuated: a dose of 60 MID50 of the parent, wild type dengue-4 virus (341750 Carib) was required to elicit seroconversion of 50% of the experimentl animals. The dengue-4 vaccine may be suitable for trials with human volunteers.